Switching devices, in particular low voltage contactors, have traditionally been used for different load switching applications and the contact material of the electrical contact tip has been chosen for these duties. However, there is an increasing demand for switching devices in applications where there are not many load switching operations. These are for instance isolation and by-pass applications.
The body of the contact tip is typically arranged of a composite material of silver metal oxide (Ag—MeO). The choice of contact tip material is a compromise between several opposing requirements, such as low contact resistance, low erosion wear and good welding properties.
It is desired to use the same contact material of the contact tip for both by-pass and isolation applications as well as for load switching applications. In isolation/by-pass applications low contact resistance to achieve low losses and hence less thermal problems is the main criteria while in load switching applications the life time of the contact tip is the most important parameter. The latter depends mainly on the erosion properties of the contact tip material.
Accordingly, a problem with prior art contact tips for use in both by-pass/isolation applications and load switching applications is that the material properties are not optimized for either of the applications.
To get a low enough contact resistance the materials in prior art contact tips contain typically 86 mass % silver, 12% tin oxide and 2% bismuth oxide. This gives a relatively good compromise between erosion resistance and low contact resistance. A harder material with less silver could give lower erosion rates but would at the same time increase the contact resistance. A softer contact material would do the opposite.
U.S. Pat. No. 4,672,008 discloses an electrical contact provided with a coating adapted to prevent formation of segregation or depletion layer on the outer surface of the contact. The thin layer could be produced by powder metallurgical sintering.
US20060239854 discloses a contact comprising an outer layer adapted to reduce the abrasion of the contact and enable the contact to be used in heavy loads. The outer layer has a higher hardness than the inner layer of the contact.